The Rise of Mumps in the United States

Earlier this week, the Hawaii Department of Health reported 6 additional cases of mumps, for a total of 636 cases statewide as of November 30th. This is a stark contrast to the much lower average yearly case count of 3.5 reported by the state in the last ten years. While Hawaii is currently experiencing an unusually large outbreak, the number of mumps cases throughout the United States has been growing at an alarming rate--from 229 cases in 2012 to 5,833 cases in 2016. As of November 4th, 4,980 mumps cases have been reported to the CDC from 47 states and the District of Columbia this year.

In today’s blog post, we look at two potential reasons that might be contributing to both the unusually large outbreak in Hawaii and to the overall increase in mumps incidence in the United States:

More susceptible population due to those who are unvaccinated

Waning immunity of those immunized

What is Mumps?

Mumps is a highly contagious viral disease that is characterized by pain, tenderness, and swelling of the parotid gland(s), which are glands responsible for producing saliva. Other symptoms include fever, headache, muscle aches and tiredness. The virus replicates in the upper respiratory tract, and is transmitted via respiratory secretions, saliva, or through fomites. Less than 1% of infected individuals develop severe complications such as pancreatitis, deafness, meningitis, and encephalitis.

Currently, there are two options for the mumps vaccine - the MMR vaccine which protects against measles, mumps and rubella, and the MMRV vaccine which protects against those three diseases as well as varicella (chickenpox). The Centers for Disease Control and Prevention (CDC) recommends children receive one dose of the MMR or MMRV vaccine at 12-15 months of age and a second dose between 4-6 years of age. The MMR vaccine is safe and effective, but does not prevent all cases of mumps. The CDC reports that one dose of the MMR vaccine is about 78% effective (range: 49-92%) at preventing mumps, while two doses is about 88% effective (range: 66-95%).

More susceptible population

The first factor that may be contributing to the growing incidence of mumps in the United States is a more susceptible population due to reduced vaccine coverage. The 1998 MMR vaccine controversy--in which researchers falsified data and failed to disclose financial interests--led many parents to believe in a false link between autism and vaccinations.This has contributed to a decrease in vaccine coverage, and is particularly concerning because, according to the CDC, “people who have received two doses of the MMR vaccine are about nine times less likely to get mumps than unvaccinated people who have the same exposure to mumps virus.”

Additionally, recent changes to public health recommendations for routine vaccination may have led to some individuals missing their second dose of the MMR vaccine. Prior to 1998 the administration of the second dose of MMR was a state-specific law, where each individual state could decide to administer the second dose either between the ages of 4 to 6 or 11 to12. In 1998, the Advisory Committee on Immunization Practices (ACIP) along with the American Academy of Pediatrics (AAP) and the American Academy of Family Physicians (AAFP) agreed on 4 to 6 years of age as the recommended age for the second MMR does. This change may have led to those planning on getting the dose at age 11/12 to miss the second does entirely.

Low community vaccination coverage against mumps can be particularly dangerous for those children who cannot be vaccinated for medical reasons, and who rely on herd immunity for protection. A study done in Canada found that vaccine coverage between 88.2% - 98% was needed for herd immunity to disrupt viral transmission, assuming vaccine effectiveness of 85% (with 2 doses) and a reproductive number between 4 and 6. According to data from the CDC, in 2016, a total of nine states reported lower than 88.2% two-dose MMR vaccine coverage for adolescents between the age of 13 – 17. These states include: Arizona, Texas, Utah, Idaho, Virginia, Montana, West Virginia, Missouri, North Carolina.

Waning immunity

Another important factor that may be contributing to the rising incidence of mumps is waning immunity. Waning immunity, according to the CDC, is the progressive loss of protective antibodies against an antigen or disease that occurs with the passage of time. A 2013 study conducted in France by Vygen et al. demonstrated that the immunity conferred by the second MMR dose decreases by 10% each year post administration.

To help prevent against waning immunity, the ACIP expert panel convened this past October and advised that a third dose of the MMR or MMRV vaccine be recommended to at-risk populations during an outbreak. The major bases behind the recommendation was a study published in the New England Journal of Medicine this past September that showed a 78.1% lower risk of mumps in those who received a 3rd dose of the MMR vaccine during an outbreak of mumps at the University of Iowa. Researchers in other countries have made similar suggestions. The study done by French researcher Vygen et al. mentioned previously contributed to the French High Council of Public Health’s decision to recommend a third MMR dose during outbreaks for those whose second dose dates greater than 10 years. Analysis done by Cordeiro et al. in 2015 also came to the same conclusion that we either need more booster shots of the actual vaccine or we need better vaccines.

To stop the growing incidence of mumps in the United States, public health practitioners must continue to stress the importance of timely childhood MMR vaccination. It is a safe and effective way to prevent disease, and also protects those who are unable to receive the vaccine themselves. Additionally, a third dose of the MMR vaccine should be made available to those at highest risk during an outbreak, which will hopefully prevent further transmission.

Outbreak Observatory aims to collect information on challenges and solutions associated with outbreak response and share it broadly in near-real time to allow others to learn from these experiences in order to improve global outbreak response capabilities.